If it's about 20, the threshold of impact for naphthanate seems to be around 11 or 12 milligrams per litre. So 20 could probably have a marginal effect if it were released into an area where there were larval fish or something like that.
The other comment I should make is that naphthanates change in structure as they age. The ones that are very young and have only just come out of bitumen and have not been exposed to the environment and microbial degradation are more toxic than the ones that have weathered for three to five years. I'm assuming at 20 milligrams per litre it is new stuff that hasn't been subject to atmospheric degradation.